Thermal control device



Aug- 1 1939 G. R. TowNsEND 2,168,180

THERMAL CONTROL DEVICE Filed April 1, 1938 Llll Inventor: George R'Vowrwsevd,

b )Va/m7 y His Attorney.

Patented Aug. l, 1939 THERMAL CONTROL DEVICE y George R. Townsend. Schenectady,

c Company, a corporation or to General Electri of New York N. Y., assign- Application April 1, 1938,v Serial No.` 199,366

11 Claims.

My invention relates to thermal control devices, more particularly to temperature responsive control devices for controlling an electric circuit in response to changes in temperature, and has for its object the provision of a reliable and eliective device of this character which 1s sensitive and quick acting, and which is adjustablefor either automatically or manually resetting the control means.

In controlling an electrical circuit in response to changes in temperautre to prevent over-heating or over-loading, th'e chief problem involved is in opening and closing the circuit quickly without detrimental arcing at the contacts. It is also desirablethat the switch be operable by a small temperature change for providing high sensitivity.

In carrying out my invention,I have provided a device whose parts are in a denite relationship with each other to obtain the required snap action. 'Ihe principle of operation isibased o n a combination of a bimetallic strip having a deiinite spring gradient working against an opposing force having a somewhat steeper gradient. This opposing force consists of the bias of a leaf spring operating through suitable leverage plus the tip pressure of a movable contact. When heat is applied to the bimetallic strip, a force is built up which finally is suilicient to overcome the opposing forces. At this point the switch starts to operate. The force of the bimetallic strip decreases in value as the switch opens, but the op. posing force decreases faster, and the switch arrives at its open position with the bimetallic force slightly greater than the opposing forces. When the bimetallicstrip is cooled sufficiently to reduce its force by this amount, the opposing force becomes stronger and the switch will start to close again. As the switch closes, the opposing force increases at a faster rate than the bimetallic force, and the switch arrives in its closed position with its opposing forces slightly greater than the bimetallic force.

For a more complete understandingof my invention, reference shouldV be had to the accompanying drawing, Fig. i of which is an elevation view of a switch embodying my invention with the cover plate removed from the casing having its contacts closed and its bimetallic element in a normal unheated position; Fig. 2 shows an elevation view partly in section with the switch opened to its automatic reset position; Fig. 2 shows an elevation view with theswitch having a portion of the cover plate removed opened to its manual reset position; Fig. 4 is a side elevation View of Fig. 3 partly in section; Fig. 5 is a side elevation (Cl. 20o-113) View of Fig. 2 partly in section; Fig. 6 shows an. elevation view of a `constructional detail `of a portion of the reset mechanism; Fig. 7 ls a perspective view of some of the members comprising the snap mechanism.

Referring to the drawing, I have shown my invention in one form in the various views as aping the cover plate II in position.v Extending downwardly through an opening I4 inthe top of the casing I0 is a heating element I5 in the form of a helical resistance Wire which is supportedA by the screws I6 and I'i threaded into the top of the casing I0 on either side of the hole I4. The

heating element I 5 is connected in series with the terminals I8 and I9 having the screws 20 and 2I adapted to secure the wires of a circuit (not shown) which will heat the element I5.

At the side of the casing I0, mounted on a projection 22 of insulating material which may be integral with the casing I0 are the terminals 23 and 24 which may be separated by a iin of insulating material 25. The terminals each have appropriate fastening means, such as screw 2B, to

rmly hold the wires o1' the incoming circuit which is to be completed or broken responsive to the heating of the element I5. The terminals 23 and 24 extend through appropriate openings into the inner portion of the casing I0 and carry therein the resilient strips 2l and 28 which are formed of copper or other suitable resilient conducting metal and have fastened at their lower ends the contacts 28 and 30 which are normally held in pressure engagement by the strips 21 and 28. The resilient strips 2.1 and 28 may be xed at the terminals 23 and 24 by any suitable means such as by welding or riveting, and the contacts 29 and 30 may be vfastened to the strips 21 and 28 in a like manner.

Spaced apart a short distance from the heating element l5 and also-extending downwardly into the casing I0 is a thermoresponsive member in the form of a. bimetallic element 3I which is made of two strips of metal having dissimilar coeflicients of expansion, such-as invar and steel,

the strips being securely pressed or welded to-` gether lengthwise. Ihe bimetallic element 3| may be somewhat triangular in shape having its broad end fixed securely at the top of the casing I by welding or brazing to the terminal strip I8 which extends over to the opening I4 and` is bent downwardly to hold the bimetallicelement 3|. The lower portion of the bimetallic element 3| is free to deect in response to the changes in temperature which occur in a manner dependent upon the current ilowing in the heating element I5. Normally, the bimetallic element 3| in its unheated condition is a straight strip, as seen in Fig. 1, which will deflect positively to the left with an appropriate rise in temperature in a manner best illustrated in Fig. 3.

The lower end of the bimetallic element 3| is loosely connected to a pivoted member 32 by a link 33 adapted to impart to the pivoted member 32 the force of the element 3| when the latter deflects in a positive direction, i. e., from right to left'. The pivoted member 32, best illustrated in Fig. 7, may be a at metallic stamping having a projection 34 at its upper end with a pair of shoulders 35 and 35 on either side thereof, the projection 34 beingv adapted to iit into an appropriate opening 31 in one face of a supporting bracket 38. The bracket 38 may be an L-shaped stamping having a hole 39 in its other face arranged to receive a rivet 40 for holding the bracket in a fixed position at the .upper left-hand corner in the interior of the casing I9 over a thickened portion of the casing wall. It may be herev noted that the rivet 40 has an eccentric body portion just under its slotted head for making a factory adjustment of the position of the bracket 38. This is best shown in Fig. 2 where it is seen that the eccentric portion of the rivet 40 is holding-the bracket 38 in a position`to the right for adjusting the pivoted member 32 in a proper relation with bimetallic strip 3| `and the floating link 49 for snap operation at a predetermined temperature. After the proper adjustment is made, the end of the rivet 40 is peaned over as shownf'in Fig. 6. 'I'he pivoted member 32 will move pivotally on the edges of the shoulders 35 and 36 which ride on the bottom of the bracket 38. 'I'he link 33 may be another metallic stamping having the projecting portions 4| and 42 at each end adapted to t loosely into rectangular openings in the bimetallic element 3| and the pivoted member 32 (the rectangular opening 43 in the pivoted member 32 is shown in Fig. 7). The loose link 33 causes the pivoted member 32 to receive the force of a positive deflection ofthe bimetallic element 3|, but when the bimetallic element 3| is caused to deflect negatively by a reduction in its temperature, no" pull is`exerted on the pivoted member 32, since the bimetallic element 3| is free to slide from left to right along the projection 42 of the link 33. V

Rigidly mounted inthe base of casing I8 by a threaded screw 44, a nut 45, and a washer 4B is a leaf spring 41 which is xed at its right end and has its left end positioned to delect in the direction of the pivot point of the pivoted member 32. The spring 41 has a slot 48 at its left end ru'nning longitudinally of the spring and a bent-over portion at its left end which includes a portion of the slot 48, so that the slot 48 runs inthe two planes of thev spring 41. For imparting the force of the spring 41 to the pivoted member 32, a floating .link 49 is provided having the V-shaped notches 59 and 5| at its upper and lowerends, which are adapted to nt into a notch 52 at the lower end of the pivoted member 32 and the slot 48 of the spring 41, respectively. vConnecting the pivoted member 32 and the resilient contact strip 21 is a lost motion link 53 which is formed of' a suitable insulating material and is adapted to permit a slight positive movement of the pivoted member 32, before the motion is carried to the resilient strip 21 for disengaging the contacts 29 and 30. The linkA 53 is slidable on the upper faces of the supports 5'4 and 55 which may be integral with the bottom of the casing I0.

Extending from the lower end of jthe pivoted member 32 is a projection 58 which cooperates with a slidable stop member 51 on' the left inside wall of the casing I8, to limit the movement of pivoted member 32 in the automatic reset position (shown in Fig. 2) or the manual reset position of the switch (shown in Fig. 3). The slidable member 51 is mounted between the left wall of the casing ||l and a guide projection 58 extending outwardly from the back wall of the interior of the casing I0. and comprises a ilat metal bar having an o-set portion at its upper end carrying at the end thereof, above the casing I8, a handle 59 of suitable insulating material. 'I'he off-set portion of the slidable member 51 extends through an opening in the upper portion of the casing |0 and through a guide collar 68 formed in the metal terminal I8. Along the rear edge of the o-set portion of the slidable member 51 is a hook-shaped projection 5| extending through an opening 52 in the rear wall of the casing l0. .The projection 5| is adapted to cooperate. with the free end of a Astraight wire spring 53 to hold the slidable member51 in either of two positions, the spring 53 being securely riveted at its other end to the casing I0 by a rivet 54. When the wire spring 53 is placed beneath the projection 5| (shown in Fig. 4), it will bias the slidable member 51 in an upper position, so that` the projection 59 on the pivoted member 32 will contact the left wall of the casing I0 in the manual reset position of the switch. When the switch is-in its manual reset position, the handle 59 may be depressed to cause the lower end of the slidable member 51 to contact the projection 55 on the pivoted lmember 32, for -forcing the switch beyond its dead center position to enable it to snap to a closed position. When the wire spring 53 isiplaced on the top of the projection 5| in the hook thereon (shown in Fig. 5), it will bias the slidable member 51 to its lower position.. so that the projection 59 on the pivoted member 32 will contact the slidable member 51, .now interposed between it and the left Wall of casing III, to prevent the switch from passing its dead center position and limit it to the automatic reset'position.

In operation, the switch is normally positioned as illustrated in Fig. 1. are closed and held in pressure engagement by Jthe resilient strips 21 and 28, and the bimetallic element 3| is in an unheated and undeflected condition. The leaf spring 41 is exerting a force through the floating link 49 which tends to rotate the pivoted member 32 in a counter-clockwise direction. When an excessive current passes through the heating element 5, heat will be radiated to the bimetallic element 3|. Thereupon, the bimetallic element 3| will build up a force tending to deflect it in a positive direction to the left; this force will act through the link 33 against the pivoted member 32 and will tend to rotate the pivoted member 32 in a clockwise direction. When this force becomes sumcient to The contacts 29 and 35l overcome the opposing force of the leaf spring 41, the pivoted member 32 and the. bimetallic element 3l will start to move to the left. As the bimetallic element deiiects to the left, the force it exerts decreases` at a given rate. The force of the spring 4"!` on the pivoted member 32 also decreases, since the leverage of the floating link 49 changes as the link approaches its dead center position, and at dead center there is no rotational force on the pivoted member 32 from the spring 4l. It should be noted, however, that the force of the spring 41 tending to rotate the pivoted member 32 decreases at a faster rate than the force of the birnetallic element 3|. After a short initial movement of the pivoted member 32, it picks up the tip pressure load of the resilient strip ,27, and this load adds to the forces opposing the force exerted by the bimetallic element 3l. The decreasing force of the bimetallic strip 3i is still sufliclent toY overcome the decreasing opposing lforces, Yand there is a snap movement of the pivoted member 32 to its automatic reset position, and the contacts 29 and .'30 open as shown in Fig. 2. In this position, the projection t and the slidable member 5l 'which is held down by the wire spring E3 cooperate to prevent the floating link 49 from passing its dead center position under the pivoted member 32. Therefore, the spring 4l still exerts a small force on the pivoted member 32 which tends to rotate this member in a counterclockwise direction against the superior force exerted by the bimetallic element 3l.

As soon as the heating element iii cools down, as it will do, if its circuit is connected to include the contacts i@ and 3d which are now opened, 'the bimetallic element 3i will also cool sufficiently to decrease its bias on the pivoted member 32 by an amount slightly greater than the diierential oi force which was holding the pivoted niember inthe last mentioned position. immediately, 'the predominating force oi the spring 4l will begin to move the pivoted member S2 in a counter-clockwise direction, and with this motion, the

. spring force on the pivoted member 32 will increase at a faster rate than the iorce oi the bimetallic element l, to cause the pivoted member to snap to its original position and enable the contacts 2@ and 'to close by 'the pressure of the resilient strips 2l and such a case, the slidable When the switch operates as inst described, it is conz'dered as ssii-resetting, or automatic resetting. For some applications it may be necessary that the mechanism does not reset itself, but that it stay open until reset manually. in

biased to the position shown in 3 by placing the wire spring benen jection di. f ed by the bimetallic element Si, its movement will carry the pivoted member over dead center position of the iioating tink d and the projection will contact 'the left a oi 'the casing leaving the contacts and open until the switch is manually reset. To reset the switch, the slidable member bl may be depressed, by pressing down on the handle i529. Ther end of the slidable member di will engage theprojection 56 and force the pivoted member 32 an appreciable distance over its dead 'center position to cause the switch to snap to its closed position and permit the contacts 29 and 30 to close.

While I have shown a particular embodiment of my invention, it will be understood, of course,

able by the force of movement of said thermoresponsive element, a spring, means including a link intermediate said spring and said pivoted member for imparting a snap action to said pivoted member after a predeterminedmovement of said pivoted member by the force of movement of said thermoresponsive element, and means operable by said pivoted member for actuating said switching means.

2.*A thermally actuable switch comprising a bimetallic element arranged for lateral deection in response to changes in temperature, switching means, a pivoted member movable by the force of deection of said bimetallic element, means including a link connecting said pivoted member and said bmetallic element for causing the movement of'said pivoted member by a deflection in only one direction of said bimetallic element, a spring, means including a second link intermediate said spring and said pivoted member for imparting a snap action to said pivoted member after a predetermined movement of said pivoted member by the force of decction of said bimetallic element, and means operable by said pivoted member for actuating said switching means.

3. A thermally actuable switch comprising a bimetallic element fixed at one end for lateral deectlon in response to changes in temperature, switching means, a member mounted on a iixed pivot point for pivotal movement by a deflection oi said bimetallic element, a leaf spring fixed at one end and positioned to impart a maximum y the force oi said positive deflection of said binietallic element, a spring, a link intermediate .said spring and said pivoted member positioned to impart a force from said spring opposing said positive deection force on said pivoted member which decreases at a faster rate than said deilection force ior giving a snap action to said pivoted member upon a positive deiiection of said bimetallic element, and means operable by said pivoted member for moving said contacts apart.

5. A thermally actuable switch comprising a bimetallic element iixed at one end for positive and negative' deflection in response 'to changes in temperature, switching means including a plurality of contacts, a pivoted member movable by a positive deection of said bimetallic element, a spring; a floating link intermediate said spring and said pivoted memberr to impart a snap action to said pivoted member from a first position to nsv a second position upon a positive deflection of said bimetallic element, means including a stop for preventing said oating link from passing its dead center position for causing a snap return of said pivoted member from said second position to said first position upon a negative deection of said bimetallic element, and means operable by said pivoted member for actuating said switching means.

6. A thermally actuable snap switch having a pivoted member, a bimetallic element fixed at one end for a positive and negative deflection in response to changes in temperature, linking means intermediate said bimetallic element and said pivoted member for causing a positive pivotal movement of said pivoted member in response to a positive deilection of said bimetallic element, a. spring, a iloa'ting link intermediate said spring and said pivoted member to impart a snap action to said pivoted member from its first position to a second position upon a predetermined positive deflection of said bimetallic element, stop means including a projection on said pivoted member and a member slidable to engage said projection for preventing said floating link from passing its dead center position for causing a snap return of said pivoted memberto its iirst position upon a predetermined negative deection of said bimetallic element, and switching means operable by said pivoted member.

7. A ythermally actuable snap switch having a pivoted member, a bimetallic element iixed at one end for a positive and negative deiiection in response to changes in temperature, linking .meansintermediate said bimetallic ele`ment and said pivoted member for causing a positive pivotal movement of said pivoted member in response to a positive deflection of said bimetallic element, a spring, a floating link intermediate said spring and said pivoted member to impart a snap action to said pivoted member from its first position to a second position upon a predetermined positive deilection' of said bimetallic element, stop means including a projection on said pivoted member and a member slidable to engage said projection for preventing said floating link from passing its dead center position for causing a snap return oi said pivoted member to its ilrst position upon a predetermined negative -,deaecn0n of said bimetallic dement, switching means including a plurality ofv contacts, and means including a link igperable by said pivoted member for actuating s'id switching means.

8. A thermally actuable switch comprising a casing of insulating material containing a bimetallic elemcnt xed at one end for positive and negative deiiection in response to changes in temperature, switching means, a pivoted member movable by a positive'deection of said bimetallic element, a spring, a link intermediate said spring and said pivoted member to imparta snap action to said pivoted member upon a positive deflection of said bimetallic element, means operable by said pivoted member for actuating said switcht ing means, and means including a member slidably mounted in said casing having a handle vportion projecting therefrom, said slidable member being arranged to engage said pivoted member for manually resetting said switchv after a suiilcient negative deflection of said bimetallic element.

9. A thermally actuable switch comprising a bimetallic element xed at one end for lateral deilection in response to changes in temperature, switching means, a pivoted member movable by a deflection of said bimetallic element, a spring, a link intermediate said spring and said pivoted member for imparting a snap action to said pivoted member upon a predetermined deection oi said bimetallic element, means operable by said pivoted member for actuating said switching means, means including a member slidable to two positions adapted for manually .resetting said- 10. A thermally actuable snap switch having a'A manual reset position and an automatic reset* position comprising a bimetallic element xed at one end for positive and negative deilection in response to changes in temperature, switching means, means including a pivoted member ladapted for movement by said positive deection of said bimetallic element, a leaf spring, a link intermediate said spring and said pivoted member for imparting a snap action to said pivoted member upon a positive deflection oi said bimetallic element, means operable by said pivoted member for actuating said switching means, means including a member slidable to two positions adapted for manually resetting said switch, and spring means comprising a straight wire spring iixed at one end with its free end adapted for biasing said slidable member to the iirst of said positions for causing automatic resetting of said switching means by limiting the movement of said pivoted member, said spring means being further adapted for biasing said slidable member to the second of said positions for enabling manual resetting o I said switch.

11. A thermally actuable switch comprising a bimetallic element fixed at one end for-positive deiiection in response to a rise in temperature, switching means including a plurality of contacts, means including a pivoted member movable by the force of said positive deection of said bimetallic element, a spring, a link intermediate said spring and said pivoted member positioned to impart a Iorce from said spring opposingI said positive deflection force on said pivoted member. which decreases at a faster rate than saidv deilection torce for giving a snap action to said pivoted member-upon a positive deection of said bimetallic element, and means includingA a lost motion link intermediate said pivoted member and said switching means for actuating said switching means after a predetermined positive 35 

